1. Field of the Invention
This invention relates to projection systems which "tile" multiple projector images on a rear projection screen, and more particularly to such a system employing a rear projection screen with improved brightness uniformity.
2. Description of the Related Art
Flat panel displays are commonly employed in lap-top, notebook or other portable computers in which packaging dimensions are of critical importance. In such applications, it is particularly important to keep the thickness of the display system to a minimum.
In prior display systems, minimum display thickness is obtained by employing a flat, minimum illumination source. The illumination source often comprises a flat fluorescent light system and a contiguous diffuser that are positioned adjacent to the rear surface of an active matrix liquid crystal module. The diffuser is utilized in an attempt to distribute light from the fluorescent source evenly over the liquid crystal module. In viewing the displayed image, the viewer effectively looks through the liquid crystal module at the diffused light source behind it.
With this method, it is very difficult to provide uniformity of brightness over a large surface area of the display. Further, because the viewer is effectively looking through the liquid crystal display at the diffuse light source behind it, light rays that pass through the liquid crystal display at an angle will traverse a longer path through the liquid crystal material than those that pass through at a normal to the display surface. This results in image contrast degradation.
The display system described in related U.S. application Ser. No. 08/040,648 avoids these problems by using a collimated light source to illuminate a liquid crystal light valve (LCLV) and a rear projection diffusing screen in front of the LCLV to diffuse the light and control the field- of-view of the display system. The use of collimated light to illuminate the LCLV avoids the contrast problems discussed above. However, the display thickness must be at least as great as the projection screen width due to the need to collimate the illumination light before it reaches the LCLV. This limits the applicability of the display system to flat panel displays with relatively small screen sizes.
One way of overcoming this screen size limitation is to display the image as a set of individual smaller image portions that are displayed side- by-side in an array fashion on the rear projection screen. Each of the smaller image portions is created with separate small image sources, such as LCLVs. This technique, known as "tiling", allows one to create a display system with an arbitrarily large screen size, while still employing a number of relatively small image sources. The problem with tiled systems is non-uniformity in brightness between the adjacent image portions on the projection screen when viewed either on or off-axis. This is because the brightness of an image that is diffused from a rear projection screen varies as a function of both the angle of incidence that the image makes with respect to the projection screen, and the angle at which the observer views the image on the projection screen. Since each of the tiled images is incident on the screen at a different angle, and since an observer views each of them at a different viewing angle, the observer will perceive brightness differences between them.